The present invention relates to a apparatus for determining the wear on mold walls during continuous casting of metals, and to the use of such an apparatus for determining the shrinkage of the casting shell from the inner wall of the mold.
In continous metal casting, the casting is produced by continuously casting liquid material through a chute in the mold. The walls of the chute, which ay be fabricated from copper, are frequently cooled to facilitate absorption of heat from the casting as it moves through the chute. When exiting from the chute, the casting has a solid shell and a liquid core. Further cooling after the casting leaves the chute results in complete solidification.
Friction between the casting and the chute walls results in wear of the walls (this occurs at the rate of about 0.5 mm in 10 hours of operation in the casting of steel slabs). If the resulting reduction in the thickness of the chute walls exceeds a system specific limit value (e.g. 10 mm), the mold must be exchanged. At present, the reduction in wall thickness is determined during intervals between castings. However the maximum casting periods have increased substantially in recent years, so that a determination of the wear during the casting process itself is of great significance. Such a determination is hindered by the "lifting" of the casting from the mold walls, which varies with time and positon within the chute. This lifting is caused by shrinkage of the cross section of the casting due to cooling of its shell upon contact with the cooled mold walls. After the shell of the casting has lifted itself away from the mold walls, it is heated again due to the influx of heat from its liquid core, causing the shell to expand until it comes back into contact with the cooled chute wall again. This process may be repeated several times in one casting cross section while it passes through the mold, with the location and amount of shrinkage in the chute varying over time. The gaps produced between the chute wall and casting as a result of the shrinkage are sometimes filled with casting powder, particularly if the shrinkage is slight. In addition to the local variations in shrinkage which produce the lifting phenomenon, there is also a progressive shrinkage as the casting continues to cool while moving through the chute. This reduces the average cross-sectional dimensions of the casting.
The reduction in cooling of the casting due to shrinkage weakens shell growth in the mold and may produce disturbances in the casting process; in particularly serious cases breaks may occur in the casting. For that reason, the mold chute is given a tapered shape, such as a conical shape if a rod of circular cross section is being cast, in order to accommodate the shrinkage of the casting as it cools while moving through the chute. The amount of taper is a critical parameter. If it is too slight, the reduction in cooling due to shrinkage may become too great because of inadequate contact with the cooling walls of the chute, while if it is too great, friction between the casting and chute walls in the lower chute region may become too great. For the majority of the casting programs, empirical values are available which permits satisfactory casting procedures. When such casting programs are used continuous checking of the wear of the chute walls, and possibly readjustment of the walls to compensate for such wear, are of great significance. For particularly critical casting tasks, however, measurements of the amount of lifting in the mold and setting of the taper according to such values are necessary. This is particularly applicable for greatly varying casting speeds. Measurement of the amount of lifting has not been possible in the past.
German Offenlequngschrift No. 3,110,012 discloses an arrangement in which a sensor below, and thus outside of, the mold determines changes in the distance to the casting surface. In dependence on these changes, the taper of the mold is adjusted during the casting process. However, this arrangement does not detect wear of the mold walls. Therefore, optimum adjustment of the taper is impossible. Moreover, problems must be expected due to expansion of the casting after it leaves the mold so that no clear distinction can be made as to whether a change in the distance between the casting surface and the sensor is the result of a change in casting shrinkage while the casting is in the mold or whether such a change is the result of deformation of the casting outside the mold.